18. c3d5  18... b7b5 A good move, breaking White's structure and trying to destabilize the knight on d5. This hint of activity bothered Ding, who inexplicably decided to simplify the position instead of fighting for the initiative. 19. c4b5 A positional mistake as the knight loses its formidable position in the center. 19... a6b5  20. d5f4 This is Ding's idea, forcing many piece exchanges.